Lithium batteries have become a useful and desirable energy source in recent years. Generally speaking lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between spaced apart positive and negative electrodes. Batteries with anodes of metallic lithium and containing metal chalcogenide cathode active material have received acceptance in industry and commerce.
By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. Cells having a metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through a liquid electrolyte to the electrochemically active (electroactive) material of the cathode whereupon they release electrical energy to an external circuit.
It has recently been suggested to replace the lithium metal anode with an insertion anode, such as a lithium metal chalcogenide, lithium metal oxide, coke or graphite. These types of electrodes are typically used with lithium-containing insertion cathodes to form an electroactive couple in a cell. The resulting cells are not charged in an initial condition. Before this type of cell can be used to deliver electrochemical energy, it must be charged. In the charging operation, lithium is transferred from the lithium-containing cathode to the anode. During discharge the lithium is transferred from the anode back to the cathode. During a subsequent recharge, the lithium is transferred-back to the anode where it reinserts. Thus with each charge/discharge cycle, the lithium ions (Li+) are transported between the electrodes. Such rechargeable batteries, having no free metallic species, are called rechargeable ion batteries or rocking chair batteries. See U.S. Pat. Nos. 5,418,090; 4,464,447; 4,194,062; and 5,130,211.
Various materials have been suggested and employed as the cathode material in the aforementioned batteries. Preferred-positive electrode active materials generally include LiCoO2, LiMn2O4, and LiNiO2. These materials are synthesized by a variety of synthesis modes which can generally be classified as “wet method synthesis”. Methods of making lithium compounds are described in U.S. Pat. No. 5,135,732 by Barbus, et al. and U.S. Pat. No. 4,246,253 by Hunter, and involve the formation of aqueous solutions as intermediate steps. Lithium compounds containing cobalt are relatively expensive to synthesize due to the intermediates required, while successful synthesis of lithium-nickel compounds is relatively complex and difficult. Lithium-manganese compounds, such as LiMn2O4, are generally more economical to synthesize than the preceding material and result in a relatively economical positive electrode.
Unfortunately all of the foregoing materials have inherent drawbacks when employed as electroactive materials in electrochemical cells. Cells employing each of the foregoing materials in the cathode experience significant loss of-charge capacity over repeated charge/discharge-cycles, commonly-referred to as cycle fading. The initial capacity available (amp hours/gram) from materials, such as LiMn2O4, LiNiO2, and LiCoO2, is less than the theoretical capacity because-significantly less than 1 atomic unit of lithium engages in the electrochemical reaction. This initial capacity value is significantly diminished during the first cycle of operation and diminishes even further on every successive cycle of operation. Thus for LiNiO2 and LiCoO2 only about 0.5 atomic units of lithium is reversibly cycled during cell operation.
Many attempts have been made to reduce capacity fading, for example, as described in U.S. Pat. No. 4,828,834 by Niagara et al. However, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing electrode material having acceptable capacity without the disadvantage of significant capacity loss when used in a cell.
In related applications, U.S. Ser. Nos. 09/204,944 and 09/559,861 which are currently pending before the United States Patent and Trademark Office, the inventors have disclosed novel lithium metal phosphate and lithium metal fluorophosphate materials which address concerns such as cycle fading and the like. However, there remains a long-felt and, as yet, unsatisfied need for providing an economical and reproducible synthesis method for such phosphate-containing materials which will provide good quality material in suitable yields.